Effect of quenching and partitioning on the microstructure and mechanical properties of a medium carbon low alloy low silicon (0.3C–1.0Mn–0.4Si–0.6Cr–0.46Ni–0.26Mo) steel

The importance of advanced high strength steel having high strength with balanced ductility has led to immense researches all over the world for increasing the strength to weight ratio in different structural applications. The results of quenching and partitioning (QP) heat treatment process on a hot rolled advanced medium carbon steel (0.3C–1.0Mn–0.4Si–0.6Cr–0.46Ni–0.26Mo) have been analyzed in the present investigation. Hardness, tensile and yield strength of the steel show considerable improvement after the heat treatment process. Microstructural analyses by Optical, Scanning and Transmission Electron Microscopes show the creation of smaller units and sub-units of multiple phases containing martensite, retained austenite and bainite in the QP steel than those of as-received steel. Microstructure evolution was assessed by conducting the heat treatment with different isothermal partitioning times followed by x-ray Diffraction phase analyses. Austenite content has been found to increase with the duration of partition. Mechanical properties were related with the microstructures obtained at different stages of partitioning. Formation of thin retained austenite films in the periphery of individual bainite and martensite strips, blocks of austenite and the presence of overall small microstructural features have been proposed to be the reason for the improvement in hardness, tensile and yield strength and ductility of the as received hot rolled steel after the designed quench and partitioning treatment.